Earthquake

An earthquake is usually caused by the rupture of a geologic fault, or the seam between two large blocks of land that suddenly move in different directions. The two predominant types of faults are thrust faults and strike-slip faults. A strike slip fault is the most common in the United States; it is where two geologic plates move in opposite directions relative to each other, such as the San Andreas in California. A thrust earthquake occurs when one plate moves under another. In 2004, a great earthquake off the Indonesian island of Sumatra was caused by a thrust fault; the rupture along the fault was greater than 93 miles (150 kilometers). The earthquake and the massive tsunami that was generated by the tsunami killed about 200,000 people in the Indian Ocean basin. In the United States and Canada, the Cascadia fault off shore of British Columbia, Washington, Oregon, and California, could potentially generate an earthquake of magnitude 9.0, and could generate a significant tsunami that could endanger people throughout the Pacific Basin. Nations subject to earthquake hazards include, but are not limited to, Indonesia, Iran, India and Pakistan, Turkey, Greece, Italy, China, Japan, Taiwan, Canada, Mexico, and the United States.

Its moment magnitude number, often mistakenly called the Richter scale that is now considered obsolete, reports the magnitude (M) of earthquakes. The moment magnitude scale is logarithmic, which means that a magnitude 5.0 earthquake (M 5.0) is about 31 times weaker than an M 6.0 quake, and is 1,000 times weaker than an M 7.0 temblor. The primary danger to people posed by earthquakes is from the structural failure of buildings due to ground shaking. A building can collapse partially or totally when the building loses structural integrity. This is more likely to happen when buildings are built on unconsolidated soils, such as sand or clay, which tends to amplify the ground motion. People are killed or injured when buildings or other structures [Page 502]collapse; the major cause of death in the Loma Prieta (San Francisco) earthquake of 1989 was the collapse of an elevated freeway in Oakland.

A great deal of experience has been amassed on how to build or locate structures to reduce the risk of building collapse in an earthquake. Engineers know how to design buildings that may be damaged in an earthquake, but retain sufficient structural integrity to allow occupants to escape. Modern steel-frame skyscrapers tend to withstand earthquakes remarkably well. The most dangerous types of buildings are unreenforced masonry buildings, and concrete and steel structures that are not built with seismic safety in mind. The 1906 San Francisco and 1925 Santa Barbara earthquakes in California revealed the problems with unreenforced masonry buildings, but the 1933 Long Beach earthquake damaged or destroyed down many masonry buildings, particularly schools. As a result, the California legislature passed the Field Act, which required that public buildings such as schools be built to withstand earthquake forces. In the years that followed, as experience accumulated with other great quakes (1964 Alaska, 1971 San Fernando), improved building codes and practices were adopted that greatly reduced the risk to human life from moderately large earthquakes.

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